Conventionally, a storage area network (SAN) is known, which is a network that connects one or more computer with one or more external storage apparatus. A SAN is often used in a case where one function is presented by coordinated operation of a plurality of computers, or in a case where one storage apparatus of large capacity is shared by one or more computers. A SAN has advantages in that it allows easy addition, deletion and replacement of storage resources and computer resources, at a later time, and thus has excellent expandability.
In a SAN, a disk array apparatus is generally used as an external storage apparatus. The disk array apparatus is an apparatus which is equipped with a plurality of magnetic disk drives, typically, hard disk devices. In a disk array apparatus, a plurality of magnetic disk drives are managed as a single group, by means of RAID (Redundant Array of Independent Disks) technology, for example. Such a group of magnetic disk drives is called a “RAID group”. A RAID group forms one or more logical storage extents. A computer connected to the SAN executes data I/O processing with respect to the logical storage extent(s). When writing data to a logical storage extent, the disk array device stores, for example, one or two redundant data to the magnetic disk drives constituting the RAID group. Storing redundant data in this way is beneficial in that, even if one of the magnetic disk drives suffers a failure, it is still possible to recover the necessary data from the data on the other magnetic disk drives of the RAID group.
On the other hand, volume shredding technology which erases all of the data in a volume is also known. In volume shredding, user data is erased by overwriting meaningless dummy data onto the whole of the volume on which the user data was written. Since this data is overwritten to the storage extent of the whole volume, the process takes a long time, and depending on circumstances, it can take from several hours to several days. For example, the U.S. Department of Defense (DoD) STANDARD, which is one set of standards relating to volume shredding, stipulates that the overwriting of dummy data is to be repeated at least three times or more. According to this volume shredding technology, since it is not possible to recover the data, then unwanted user data can be erased completely, which is advantageous from a security viewpoint.
In general, there are limitations on the number of logical storage extents which can be associated with the ports of a storage system, and hence there are limitations on the number of logical storage extents that can be accessed from a host computer. On the other hand, in dynamic volume allocation technology (see, for example, Japanese Patent Application Publication No. 2005-209149), the logical storage extent associated with a device subject to access from a host computer is changed in accordance with a request from the computer. This technology is advantageous in that, regardless of the number of ports belonging to the storage system, or the number of logical storage extents that can be associated with any one port, access to a plurality of logical storage extents can be permitted and hence the use efficiency of the logical storage apparatus can be improved.
Moreover, in a conventional SAN operation, when mounting a storage volume on the file system running in the host computer, it has been necessary to allocate the physical disk capacity of the corresponding storage volume, statically, in advance. However, in a SAN operation of this kind, expanding capacity or creating or deleting volumes requires a large amount of work, such as system shutdown, and the like. Therefore, a “Thin Provisioning” technique has been developed, in which, rather than allocating a physical disk capacity statically in advance, virtual volumes, namely, volumes in the form of virtual storage extents are presented to the host computer, and a logical storage extent is allocated dynamically from a storage resource pool whenever a write operation from the host computer arises (see, for example, Japanese Patent Application Publication No. 2003-015915). The previously defined storage resource pool may be composed with a relatively small capacity compared to the virtual volume, and in addition to improving the use efficiency of the storage capacity, it is possible to expand the capacity of the storage resource pool without affecting the host computer, and therefore, operation is simplified and the management burden is reduced.
As described above, in a complete erasure (shredding) process relating to a storage apparatus, such as a magnetic disk drive, since dummy data is overwritten to the whole extent of the storage volume, the process may take a very long time, for instance, from several hours to several days. Consequently, a problem arises in that a host computer using a volume that is the object of erasure cannot carry out read or write operations from or to the volume concerned, until the shredding process for that volume has completed, and this produces a very long waiting time.